<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<input type="text" id="input">
<button id="btn"> 验证</button>
<script>
var s = "abcAFdd";
var s1 = s.toUpperCase();
console.log(s1);
console.log(s.toLowerCase());

btn.onclick = function (){
    var val = input.value;
    if("abc"==val.toLowerCase()){
        alert("输入正确")
    }else{
        alert("输入错误")
    }
}

</script>
</body>
</html>